Best answer by Airgetlam
Does your receiver put out a Dolby Digital signal on the RCA port you're planning on using? That's the only way that the PLAYBAR would send a signal to the SUB.
Seems like your chances of making this work are close to nil, but I could be wrong.
But, the second half would work. You can indeed pair a SUB with a PLAYBAR and play music using Sonos' software.
The PLAYBAR is not a center speaker, no. It's a front right, center and left speaker, which interprets a Dolby Digital signal and sends the data to the appropriate speakers, which are inside the soundbar.